CHAPTER XXVI 

 MICRO-ORGANISMS IN RELATION TO SOIL FERTILITY 



JACOB G. LIPMAN 



New Jersey Agricultural Experiment Station, New Brunswick, N.J. 



The average cultivated and improved soil is a good culture medium for higher 

 plants and for micro-organisms. A soil is said to be fertile when it manifests ability to 

 produce large crops throughout a series of years. By the same token it is infertile 

 when its crop-producing power is below the average. But a soil fertile or infertile for 

 higher plants is usually also fertile or infertile for micro-organisms. This is borne out 

 by widespread experiments and observations which provide ample proof that an inti- 

 mate relation exists between the macrofiora of soils, on the one hand, and their micro- 

 flora and fauna, on the other. 



THE NATURE OF SOIL FERTILITY 

 INTERNAL FACTORS 



A soil consisting, for the most part, of minute fragments is said to be "fine- 

 textured soil," and when made up of particles of relatively large size, it is designated 

 as a "coarse-textured" soil. The relation of the fineness of soil material to texture may 

 be well illustrated by the figures given by Lyon, Fippin, and Buckman, who note that 

 for the average gram of material there will be found, in coarse sand, 1,670 particles; 

 in fine sand, 131,900 particles; in very fine sand, 1,676,500 particles; in silt, 35,934,000 

 particles; and in clay, 45,632,000,000 particles. Thus soil texture must necessarily 

 affect the rate of movement of water and air, the temperature, microbiological activi- 

 ties, and the concentration and composition of the soil solution. Obviously, then, the 

 different factors just noted exert a reciprocal influence on one another. 



Wide differences exist in the chemical composition of soils. Their origin, their 

 age, and the environment under which they were formed all serve to determine their 

 value as a culture medium. Sandstones, limestones, feldspars, granites, and other 

 rocks when exposed to weathering will give rise to soils of different physical and 

 chemical quality. In humid climates the soil is exposed to constant leaching. Basic 

 ions such as calcium, magnesium, potassium, sodium, and iron are gradually made 

 soluble and carried away. Given a time long enough, everything except the silica 

 skeleton of the soil may be more or less thoroughly removed. In the earlier stages of 

 the process the loss of basic material may not be as clearly reflected in the vegetation, 

 but sooner or later the increased hydrogen-ion concentration is certain to affect the 

 crops, as weU as the soil micro-organisms. On the other hand, in arid and semi-arid 

 regions the leaching of the soil is less thorough, an acid reaction does not develop so 

 readily, if at all, the soil tends to become basic in character, and not infrequently 

 an undue accumulation of soluble salts (known as alkali) takes place. Hence the 

 composition of the parent-rock, the length of the weathering period, and environ- 

 mental factors of temperature, rainfall, glaciation, and aridity have always played a 



341 



